Bracing 1/4" plywood

Sundogue · #1 12-13-2010, 07:04 AM

I'm thinking of trying to build a cabinet (a 110 or 210...not sure yet) out of 1/4" plywood.

Anyone have some suggestions on what would be the best way to brace it? How far apart do you think the bracing would need to be to make it solid and prevent resonance and vibrations from adversely affecting the sound?

I'm sure it can be done and it's all in the engineering. The edges would have 1X1 strips glued in for attaching together (no I'm not going to try and glue and screw into the edge of 1/4" ply

).

Thoughts?

T-Bird · #2 12-13-2010, 07:49 AM

Hi.

I'm no expert on light weight plywood construction, but as an educated (BEng Mech.)guess, I'd say that the weight saving will be around 10% compared with less braced 1/2" construction, and at least twice the work.

The only remotely feasible method I can think of is to build a sandwich cab out of 2*1/8 plys + 1" stiff foam. If glass fiber is not an option.

Regards
Sam

Wes Whitmore · #3 12-13-2010, 08:11 AM

Bgavin has done a lot of work with 3/8" Baltic birch. I'm sure he will chime in.
Wes

rpsands · #4 12-13-2010, 09:10 AM

I'd aim for no unbraced panel sections more than 4".

Savings should be closer to 1/3rd of the wood weight of 1/2" ply. Especially attractive for large single driver cabinets like fEarfuls (where wood weight makes a big difference).

Given how flexible 1/4" ply is, I would consider stress bracing it (e.g. crucifix style bracing like Andy from Acme uses) at certain points before installing ribs. Front to back, side to side, top to bottom. The front to back one obviously last.

greenboy · #5 12-13-2010, 09:22 AM

"Stress bracing" on 1/4? You'll just end up with panel-wide deformation. There's not enough torsional / lateral resistance.

You really need to go Monocoque. You need both rigidity on a more global basis, and finer grained damping.

I think 3/8" is about where the fun ends and the grief begins if any driver especially high-force is going to be inside. I'd look at Petebass / zac methods instead.

greenboy · #6 12-13-2010, 09:26 AM

This is 1/4" and 3/8" by the way: http://www.talkbass.com/wiki/index.p...enboy_Headcase

rpsands · #7 12-13-2010, 09:46 AM

Based on my experiences with cheap 3/8" ply, I believe high plycount birch 1/4" would benefit from having the panels stressed.

I'd also recommend using a 1/2" baffle. Strongly.

Note: 3/4" poplar dowels is how I would glue and screw everything together. They hold screws pretty well and are cheap and light. Put them in the corners and use'em as a frame. Worked really well for me. Also, it's nice for screwing the baffle to so you can pull it on and off as needed.

Also consider brad nails as opposed to screws - should work fine as long as you get nice quality ply. They're lighter and less trouble. Do not use brad nails on cheap plywood though, unless you know enough about the tool to keep it from popping them right through the wood. Arauco lighter than 1/2"...you can shoot brad nails right through it

With 1/4" actually, 5/8" or so staples might work =P

greenboy · #8 12-13-2010, 09:51 AM

Even on small panels of 1/4" if I apply stressing I can easily deform them to the point they visibly bulge. Not only that but it's tough to get the bulge to be uniformly distributed. Which if you want things to sit level and not have torisonal twists when trying to stress on two or all three axis, is a mess.

Stressing works only when the shell material has enough resistance to "fight back".

rpsands · #9 12-13-2010, 09:57 AM

I'll bow to experience there. I haven't done it with 1/4

Munjibunga · #10 12-13-2010, 10:28 AM

I'd brace it by laminating another piece of ¼-inch plywood to it.

billfitzmaurice · #11 12-13-2010, 10:33 AM

Quote:

Originally Posted by Sundogue

How far apart do you think the bracing would need to be to make it solid and prevent resonance and vibrations from adversely affecting the sound?

Four inches.

Quote:

I'd say that the weight saving will be around 10% compared with less braced 1/2" construction

Closer to 30% actually.

Sundogue · #12 12-13-2010, 12:56 PM

Thanks for all the replies (as well as the dialog back and forth

), and even the negative responses to not even attempt it.

I love it!!!

Experimentation is half the fun of DIY, so I plan on having some fun and nothing gets me going like someone telling me not to (even when I'm flat out wrong!).

I was thinking about stress bracing (ala Acme) and I think if the bracing was against another brace (as opposed to directly stressing against the 1/4" plywood) there would be more uniformity to the bowing affect.

I've also considered laminating 1" thick foamboard to the inside of the 1/4" ply (not sandwiching in between...just a lamination) but using 3/4"X3/4" or 1X1 strips glued to the edges of the plywood for the actual construction. I would think that laminating it to one side would increase the 1/4" ply's strength more than none at all, albeit not as much as sandwiching it. Of course sandwiching two sheets of 1/4" is pointless as I would be better off using 1/2" ply by itself (well, duh).

I wouldn't even consider using 1/4" for the baffle...1/2" at minimum.

I'm also considering using 1/2" for the baffle and the back, but 1/4" for the top, bottom and sides. Is there more pressure on the front and back of a direct radiating cab than the sides, top and bottom? Curious. I would think there might be, but perhaps the pressure is equal to all walls. Thoughts?

This thread is related to my 110/6 build idea using BP102's. Currently I'm using a 210/6 cab and I love it, but I'm wanting two smaller and/or lighter 110 cabs with the same output, but half the weight/size. Although the speakers are about ten lbs. each I might be able to build a fairly light 110/6 with another light 110 extension cab. Utilizing the ability of the BP102's to put out some decent low end for a ten I might be able to build sufficiently sized cabs to produce that, without the additional weight.

At the very least, I can maintain the frequency response I currently have, build them smaller and have them even lighter still.

It all depends on how worthy 1/4" ply is for a cab. We all know commercial cabs still use 3/4" for ease of manufacture ($$$) and that building out of 1/4" is time consuming, but so what? It's my time and an enjoyable hobby, so this is just something that would be a fun venture to take a shot at. They say time is money and while some wouldn't "waste" their time doing something like this, some also waste a lot of their time staring at a TV too with nothing to show for it.

Keep the thoughts, ideas and suggestions coming. I appreciate the feedback thus far.

greenboy · #13 12-13-2010, 01:03 PM

Quote:

I was thinking about stress bracing (ala Acme) and I think if the bracing was against another brace (as opposed to directly stressing against the 1/4" plywood) there would be more uniformity to the bowing affect.

Just a slight comment, rather than going into this much: it really pays to consider what the goal of something is, rather than just doing it because in another context it seemed to make sense at the time.

Is the goal here to BOW something against something else? Or is it to achieve good global rigidity and sectional damping on the actual OUTER PANELS?

I think the way to proceed is to consider whether something meets goals. So one needs to have a clear vision of what the actual goals might be.

Sundogue · #14 12-13-2010, 01:04 PM

Quote:

Originally Posted by greenboy

Even on small panels of 1/4" if I apply stressing I can easily deform them to the point they visibly bulge. Not only that but it's tough to get the bulge to be uniformly distributed. Which if you want things to sit level and not have torisonal twists when trying to stress on two or all three axis, is a mess.

Stressing works only when the shell material has enough resistance to "fight back".

If you only used three rubber feet on the bottom of the cab (two spread along the front where the amp sits and only one in back), you'd essentially have a tripod, thereby negating the ill effects of twisting causing it to not sit level.

As far as visible bulging, perhaps it need not be stressed to the point of doing so? Or as I mentioned previously, if it were stressed against another brace as opposed to directly against the 1/4" ply?

Sundogue · #15 12-13-2010, 01:08 PM

Quote:

Originally Posted by greenboy

Just a slight comment, rather than going into this much: it really pays to consider what the goal of something is, rather than just doing it because in another context it seemed to make sense at the time.

Is the goal here to BOW something against something else? Or is it to achieve good global rigidity and sectional damping on the actual OUTER PANELS?

I do a lot of things where it simply made sense at the time.

My point was that if there is a brace glued to the back panel of the cab, couldn't the "bowing" brace push against this entire back panel by pushing on the glued brace instead of directly pushing against the thinner, weaker 1/4" plywood? I mean if there is going to be a brace there anyway, why not use the force against that instead? The brace would be a part of the outer panel at this point, not a floating stick of wood not adhered to the outer panel itself.

greenboy · #16 12-13-2010, 01:10 PM

What was perhaps a solution for Acme at that time might not even be used by Andy any more. But in either case, is it even relevant here? He had thick walls which would allow a lot more pre-stress force. The resistance mattered, and the amount of force applied against it mattered.

Think too about a very floppy spongy-wood bass neck. Visualize an effective truss rod for such an ineffective neck. The truss rod not only needs to resist the strings (and various sets of different gauges), but also provide torsional and lateral stability in varying humidities, and do so whether the string sets are especially balanced or not.

rpsands · #17 12-13-2010, 01:11 PM

If you're going to use foam core, use 1/8" ply. The BB 1/8" ply is still 3-ply, so totally legit.

An idea I had that I haven't had the chance to work on is sheet ABS over foam on the inside and 1/8" bb on the outside, but I'm not sure that would be much stronger than 1/4" ply would. But it would probably require less bracing.

Sheet ABS seems to have pretty good tensile strength, and glued to foam core I think it would be good enough. Especially if you braced even a little, maybe using plastic strips or something.

Sundogue · #18 12-13-2010, 01:28 PM

Quote:

Originally Posted by greenboy

What was perhaps a solution for Acme at that time might not even be used by Andy any more. But in either case, is it even relevant here? He had thick walls which would allow a lot more pre-stress force. The resistance mattered, and the amount of force applied against it mattered.

Think too about a very floppy spongy-wood bass neck. Visualize an effective truss rod for such an ineffective neck. The truss rod not only needs to resist the strings (and various sets of different gauges), but also provide torsional and lateral stability in varying humidities, and do so whether the string sets are especially balanced or not.

Well, it was just a thought. Stress bracing does introduce some strength. Although you are right, it does require some opposing force to be effective.

Sundogue · #19 12-13-2010, 01:32 PM

Quote:

Originally Posted by rpsands

If you're going to use foam core, use 1/8" ply. The BB 1/8" ply is still 3-ply, so totally legit.

An idea I had that I haven't had the chance to work on is sheet ABS over foam on the inside and 1/8" bb on the outside, but I'm not sure that would be much stronger than 1/4" ply would. But it would probably require less bracing.

Sheet ABS seems to have pretty good tensile strength, and glued to foam core I think it would be good enough. Especially if you braced even a little, maybe using plastic strips or something.

I've admired the Zac-Lite cabs and while I have no desire to work with fiberglass, the idea of using foamboard is appealing. I'm wondering if I were to make each panel out of 1/4" ply with 1" strips all around the outer edges (for screwing into) except where the 1/4" would overlap on joints and then laminate 1" foamboard to the 1/4" ply (tightly inside the strips) if it would make for a more rigid, yet lightweight panel even without the sandwiching effect.

rpsands · #20 12-13-2010, 01:42 PM

I'm not sure why you would use 1/4" plywood and foamboard though. It wouldn't be any lighter than 3/8".

I did some crunching on this and to use ply and foam board you really need to be down in the 1/8" range. The foam board and glue has a non-zero weight.

The answer here is to use clamps and staples to hold the glue together. You do not need to screw a cab together. PL premium on the wood parts and some kind of glue on the foam, and then some frame structure inside, should be sufficient.